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Potassium nitrate (KNO3) has seven polymorphs in the pressure range 0-4.0 GPa. The authors have studied it at room temperature as a function of pressure up to 9.3 GPa and have confirmed the structure of phase IV previously refined by neutron diffraction at 0.36 GPa. The compressibility measured in the range 0.3-9.3 GPa is found to be anisotropic with the axial compression ratios a:b:c=1.0:0.64:0.50. The relative merits of X-rays and neutrons for high-pressure studies are discussed.

Phase transitions have been observed in mercury (II) cyanide near 2.5, 89, 19 and 60 kbar. The I/II transition appears to be of second order with the structure of phase II closely related to that of the parent molecular type. Using Landau theory, three possible structures have been identified for the new phase II, of which S24 is marginally better in accord with the evidence. A major reconstruction of the lattice occurs at the II/III transition; the coordination number of mercury appears to increase to four, and the structure becomes covalently bonded, rather than molecular. Phase IV has a spectrum consistent with it having the cubic anti-cuprite structure adopted at STP by Zn(CN)2 and Cd(CN)2. Above 60 kbar, brown polymeric Hg(CN)2V is formed irreversibly. It has a single IR absorption

Raman spectra of CH3Hg (X=Cl,Br,I) have been studied as a function of pressure using diamond anvil cells. Phase transitions were discovered in the chloride at 1.5+or-0.3 and 5.9 kbar, with an almost immediate further transition near 6 kbar to a phase stable to at least 40 kbar. CH3HgBr shows a single transition at 12.5 kbar. Soft modes were found for all three materials. That for the bromide was followed by change of temperature as well as of pressure. The crystal structure of CH3HgBr was found to be like that of the chloride, but of a different space group (P421m, Z=4), allowing slight nonlinearity of the C-Hg-Br skeleton. On the basis of this structural evidence a phase diagram is proposed. A very narrow (about 0.1 kbar) phase in CH3HgCl may be incommensurate. Preliminary studies of CH3HgI show that its behaviour parallels that of its congeners, but that the crystallography is different in detail.

Infrared reflectance spectra are reported for oriented single crystals of MAMC and PAMC with the electric vector parallel to the anionic sheets for temperatures from 50 to 295K. Single-crystal Raman spectra were also obtained at similar temperatures. The main features of the spectra, particularly for the ambient temperature phases, are assigned with the aid of a factor group analysis. This phase for MAMC, although orthorhombic, behaves spectroscopically as pseudo-tetragonal, but PAMC is plainly orthorhombic. Modes above 260 cm-1 are attributed to cation internal modes, including torsions. The phase III (tetragonal) to phase IV (monoclinic) transition is distinguished in the Raman spectra by disappearance of an intense broad features at about 35 cm-1/.

Raman spectra have been obtained for HgCl2 to 145 kbar and HgBr2 to 100 kbar and phonon pressure dependencies established. The phase transition in HgCl2 said to be at 6.5 kbar, on NQR evidence, has been located at 2.2 kbar. A new first-order phase change has been found in HgCl2 at 79.0 kbar. Also in HgCl2, there are two bands that pressure scan differentially such that one overtakes the other, but they do not appear to be in resonance. Improved values for the phase transitions in HgBr2 are 4.0, 26.0 and 41.0 kbar. It is suggested that there are no structural equivalences between the two series of four polymorphs based upon the chloride and bromide respectively.